1. Field of the Invention
This invention relates generally to telephony and, more particularly, to a processor based, subscriber programmable add-on for a dial pulse activated telephone switching office to enable the office to provide various modern special services on a subscriber-by-subscriber basis.
2. Description of the Prior Art
Early in the history of telephony, it became apparent that it would be physically and economically impossible to serve the rapidly growing number of telephone subscribers with the then existing network of manual switchboards. This thinking lead to the invention and implementation of the first commercially successful automatic telephone switching system, the so called step-by-step system. The step-by-step system is a direct, progressive type of switching system that is controlled by dial pulses generated at the subscribers' telephone subset in establishing connections to other subscribers through the telephone network. At the heart of the system is a two-motion electrochemical stepping switch commonly known as the Strowger switch in honor of its inventor. The Strowger switch as a form of cylindrical symmetry with wiper contacts that, in response to dial pulses, are advanced in increments vertically and angularly to interconnect sets of terminals. Because the contacts of the individual switches involved in a call connection hold their advanced positions during the course of the call, the step-by-step system had the advantage not only of being automatic, but also of providing a memory for the switching path once the connection was made. This, combined wih advantages in terms of improved reliability, increased capacity and lower cost as compared to the manual network, made the step-by-step system the most prevalent of new telephone switching systems installed during the four decades spanning the 1920's to the 1960's.
As technology advanced and telephone demands grew, other more flexible, higher capacity switching arrangements were introduced. The crossbar switch represented a major advance in telephone switching. The crossbar, as its name implies, involves a mechanical matrix of selecting bars, oriented in horizontal and vertical rows, which can establish electrical connections at any of its various crosspoints in response to the operation of relay-type control mechanisms. Crossbar switches, like stepping switches, are capable of holding a switch path once a connection is made therethrough and, at first were used in direct, dial pulse controlled systems, in effect replacing stepping switches. As time progressed, however, the crossbar switches were used to implement the common control concept. In a typical common control system, common circuitry is provided which receives and temporarily stores the dial pulses from a subscriber subset and translates the pulses into control signals that operate the appropriate crosspoints in a crossbar switch to establish the desired connection. Once the connection is made, the common circuitry stores the switch path in a memory separate from the crossbar switch and is then available to operate other crosspoints for other calls.
Common control systems of the type are inherently capable of handling higher call densities than step-by-step systems and thus enabled the telephone companies to meet ever increasing telephone demands, particularly in densely populated areas. Common control systems also enabled the telephone companies to provide new and improved services, such as direct distance dialing and extended area dialing, that could not be provided by conventional step-by-step systems.
As technology continued to advance and telephone demands continued to grow, both in terms of numbers and in terms of the scope of services desired, modern day electronic switching systems evolved. These electronic systems also rely upon the common control concept but typically involve the use of extremely high speed, high density electronic crosspoint switches. The more advanced electronic systems also include a full scale computer that not only stores switch path information but also supervises and controls the entire switching arrangement. The use of a programmed computer to control switching operations contributed great flexibility to switching since the stored program offered a changeable memory and logic. This flexibility enabled the telephone companies to offer subscribers many new special services, such as call conferencing, call forwarding, abbreviated dialing and the like, that they were not able to offer with the more conventional systems.
Today, almost all new switching systems being installed by the telephone companies are of the electronic common control variety. There are few, if any, new step-by-step systems being installed. Nevertheless, the step-by-step system is still dominant in the total number of switching offices presently in use. Because of the recognized superiority of electronic systems in tems of their higher capacities and their ability to reliably and economically provide diverse special services to subscribers, it is likely that most presently existing step-by-step systems will some day be replaced by electronic systems. It would, however, be economically impractical for the telephone companies to engage in a wholesale and rapid changeover to electronic systems since many existing step-by-step systems have significant remaining useful lifetimes. Additionally, many of the geographical areas now served by step-by-step systems have subscriber densities that are too low to justify the expense of a replacement. As a result, it is the general opinion of knowledgeable telephone people that a changeover to an essentially all electronic switching network will not occur for at least a few decades. It is evident that, in the meantime, unless practical, economical and flexible add-ons are developed for augmenting the capabilities of step-by-step offices, subscribers connected to such offices will be deprived of the special services and other advantageous features provided by modern electronic systems.
Special services add-ons for step-by-step switching offices have been proposed heretofore. See, for example, U.S. Pat. Nos. 3,626,107; 3,710,033; and 3,997,731. The add-ons disclosed in the cited patents are disadvantaged, however, because each is capable of providing at best only two special services, e.g., incoming call waiting and call conferencing. The disclosed add-ons are further disadvantaged in that each requires rather involved modifications and equipment additions to the telephone switching office for its implementation. The add-ons require, for example, the addition of an auxiliary line circuit for each subscriber who is to receive a special service and/or the addition of a second and separate line appearance for each such subscriber line in either the linefinder equipment or connector equipment or both of the office. The noted disadvantages of these prior add-ons and others like them have militated against their widespread use and acceptance.
A system capable of providing a variety of special services to subscribers served by step-by-step and other non-program controlled switching offices is disclosed in U.S. Pat. No. 3,546,393. Thatsystem, however, is not in the form of an add-on to conventional office, but rather involves the use of a full scale, supervisory switching office which is shared by a plurality of conventional offices and which, in turn, is operated under the control of a separate stored program switching office (TSPS center). The system is thus large scale and expensive, and its cost may be justified only if a sufficiently large number of conventional offices are served thereby.
U.S. Pat. No. Re28,337 discloses a program controlled, special services add-on for a telephone switching system that is capable of providing a variety of special services. The add-on is, however, specifically adapted for use with a common control telephone switching office. It is not applicable to direct, progressive type dial pulse actuated switching offices.
There thus exists a real need in telephony for an economical, practical and flexible special services add-on specifically adapted for use with dial pulse actuated switching offices.